The answer depends on the units used for 21x19: inches, metres, kilometres? Since that information has not been provided it is not possible to answer the question sensibly.
21 x 19 feet is 399sq.ft.
You just need to multiply length times width to get the area. This will work out to 399 square feet.
96 square feet
About 1,400 square feet. (1,399.30835 square feet).
171 square feet.
10.764 square feet.
10.764 square feet.
how many square feet in 25 feet by 6 feet
10279 square feet.
2178 square feet.2178 square feet.2178 square feet.2178 square feet.
About 430.5 square feet. (430.556417 square feet)
How many square feet are in the world?
96 square feet
3.472 square feet.
About 1,400 square feet. (1,399.30835 square feet).
7500 square feet
10800 square feet
12.36 square feet
171 square feet.